Sunday, November 06, 2011

Fresh Water from Salt: The Environment of Desalination

A couple of weeks ago I had the privilege of visiting Doha, Qatar, in connection with an engineering ethics workshop at Texas A&M University Qatar. Qatar is a small nation on a peninsula in the Arabian (Persian) Gulf, almost destitute of natural resources except for oil and gas. These they have in abundance, and exchange for the more mundane necessities of life such as food and water. Especially water.

To support a population of 1.7 million, water desalination plants take seawater from the Gulf and deliver freshwater to the cities and towns, including the capital Doha. I have no statistics on Qatar’s per-capita water use, but Saudi Arabia’s is below the world average. From my very limited perspective, I did not see much in the way of water extravagance in Doha. The city was largely devoid of outdoor greenery except for small patches of grass in front of a few hotels. The larger private homes had gorgeous rose bushes climbing their back walls here and there. But by and large, it’s easy to tell that Doha is built in the middle of a desert.

Desalination is not without its environmental problems. There are two main technical processes in commercial use today: flash evaporation and reverse osmosis. The older flash evaporation method runs the salt water into a partial vacuum, which lowers the boiling point below what it is normally (around 100 C or 212 F). By arranging several stages with heat exchangers and varying pressure between stages, the flash evaporation process can be made fairly economical. But it is still energy-intensive and much more costly than any other kind of water treatment for freshwater sources.

About forty years ago, a lower-energy approach called reverse osmosis was commercialized. Crudely speaking, the salt water is sent through an osmosis membrane with pores so small that even sodium and chlorine ions can’t make it through. Regular “forward” osmosis causes water to flow from a region with lower ion concentrations to higher concentrations, with a resulting pressure difference across the barrier. To make the process run backwards, very high pressures are applied—upwards of tons per square inch. But everything happens more or less at room temperature, and high-pressure pumps use less energy per liter of water than the flash-evaporation technology does.

Both methods produce waste in the form of highly concentrated brine that is typically pumped back into the sea. If this is done carelessly, the dense brine (which also has anti-scaling chemicals added) stays on the sea floor and can harm or kill a wide variety of animal and plant life. At added expense, the brine can be diffused slowly through a large network of perforated pipes so that it doesn’t build up excessively in any one place. Or if there is a large flow of ordinary seawater available from the cooling system of a power plant, for example, the brine can be first diluted with the larger volume of seawater and then released. The second alternative makes sense if the desalination plant is operated in a “cogeneration” fashion, that is, by using waste heat or energy from a power plant (either nuclear or conventional fueled).

So far, desalination has not become a mainstream method of water production except in places that have enough cash to afford it, and don’t have other alternatives for freshwater resources. In the U. S., for example, which has the highest per-capita consumption of water in the world, we also have enough freshwater sources (so far) to avoid desalination altogether, except for a few special situations in California, Florida, and Texas.

Lack of clean drinking water is one of the major challenges in the path of improving the quality of life for billions of poor people around the globe. Unfortunately, unless a population is fairly close to the ocean and not too high in elevation, desalination is usually too expensive to be considered in comparison to simply piping freshwater from somewhere else. And it is not a technology that works well on a small scale, although I am aware of a few experimental projects which tried to develop household-size desalination plants. Even if they work, they are not as efficient as the larger units and produce a relatively large waste stream that has to be dealt with somehow.

As population pressures increase the demand for freshwater supplies, desalination should be considered as one of many options, including conservation, more intelligent utility pricing, and cooperation between private and public entities. In the U. S., the era of grand publicly-financed public works such as dams and aqueducts is mostly over. For one thing, most of the good sites for dams have dams on them already, and for another thing, the political climate in which large areas of land could be taken by eminent domain no longer exists in most places. If an environmentally friendly way can be found to power desalination plants (solar comes to mind), perhaps it will be a viable way to deal with future water crises. Here in central Texas, we are enduring the second year of a severe drought, and we’re considering all kinds of oddball ideas. But I hope the drought breaks before we have to ship desalinated water in from Galveston or somewhere.

Sources: I referred to the Wikipedia articles “desalination” and “reverse osmosis,” and a news article by Emmanuelle Landais in the Gulf News on desalination plants in the Arabian region at

1 comment:

  1. I am curious to know if the use of tidal or wave power has been used as cogeneration